Pig iron production
Pig iron is produced by smelting iron ore in blast furnaces or by smelting ilmenite in electric furnaces
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Iron ore – blast furnace
More than 1.1 billion tonnes of blast furnace iron was produced globally in 2016. Blast furnace economics are such that larger units have lower unit production costs, hence there has been a trend to bigger and bigger furnaces. Modern blast furnaces produce more than 10,000 tonnes of pig iron per day.
The blast furnace is a counter-current gas/solids reactor in which the descending column of burden materials (coke, iron ore and fluxes/additives) reacts with the ascending hot gases. The process is continuous with raw materials being regularly charged to the top of the furnace and molten iron and slag being tapped from the bottom of the furnace at regular intervals.
Modern blast furnaces can produce more than 10,000 tonnes of pig iron per day
Blast furnace process
Key steps of the process are as follows:
– upper part of the furnace
free moisture is driven off from the burden materials and hydrates and carbonates are disassociated.
– lower part of the blast furnace shaft
indirect reduction of the iron oxides by carbon monoxide and hydrogen occurs at 700-1,000°C.
– Bosh area of the furnace where the burden starts to soften and melt
direct reduction of the iron (and other) oxides and carbonization by the coke occurs at 1,000-1,600°C. Molten iron and slag start to drip through to the bottom of the furnace (the hearth).
Between the bosh and the hearth are the tuyeres (water cooled copper nozzles) through which the blast – combustion air, preheated to 900-1,300°C, often enriched with oxygen – is blown into the furnace. Immediately in front of the tuyeres is the combustion zone, the hottest part of the furnace, 1,850-2,200°C, where coke reacts with the oxygen and steam in the blast to form carbon monoxide and hydrogen (as well as heat) and the iron and slag melt completely.
Molten iron and slag collect in the furnace hearth. Being less dense, the slag floats on top of the iron. Slag and iron are tapped at regular intervals through separate tap holes. For merchant pig iron production, the iron is cast into ingots; in integrated steel mills, the molten iron or hot metal is transferred in torpedo ladle cars to the steel converters. Slag is transferred to slag pits for further processing into usable materials, for example raw material for cement production, road construction, etc.
The principal reactions are:
2C + O2 → 2CO
C + H2O → CO + H2
CO2 + C → 2CO
3Fe2O3 + CO → CO2 + 2Fe3O4
Fe3O4 + CO → CO2 + 3FeO
FeO + CO → Fe + CO2
When charging the blast furnace, burden materials are added in layers.
Charging is done either with an elevator in which a bucket is lifted up and set down on the top of the furnace to be emptied directly into the furnace (bell system) or by conveyor belts to the top of the furnace where materials are charged into a bin fixed to the top of the furnace (bell-less system) and from there into the furnace.
By means of a rotating chute it is possible to achieve very uniform distribution of the charge materials across the furnace. The bell-less system has the additional advantage that less energy rich blast furnace gas is lost during charging.
The additives and fluxes serve to convert the waste or gangue materials in the charge (mainly silica and alumina) into a low melting point slag which also dissolves the coke ash and removes sulphur. For example:
CaCO3 → CaO + CO2
CaO + SiO2 → CaSiO3
FeS + CaO + C → CaS + FeO + CO
The blast furnace itself is a steel shaft lined with fire resistant, refractory materials. The hottest part of furnace – where the walls reach a temperature >300°C – are water cooled. The whole structure is supported from the outside by a steel frame.
The blast furnace gas that leaves the top of the furnace is a mixture of carbon dioxide, carbon monoxide, hydrogen and nitrogen and has a calorific value between 3,200 and 4,000 kJ/m³. After cleaning, it is used for a variety of purposes, including heating of the hot blast stoves (“cowpers”), in iron ore agglomeration plants and for electricity generation. The credit for this gas is an important factor in keeping blast furnace operating costs low.
Ilmenite – electric furnace
Producers of titanium dioxide slag in Canada, South Africa, and Norway smelt ilmenite, a weakly magnetic ore of titanium dioxide and iron oxide (FeTiO3) in electric furnaces at temperatures ranging from 1650 -1700° C, using either coal or another carbonaceous material as reductant.
Titanium dioxide slag is the principal product, with pig iron being a co-product. Titanium dioxide slag is largely used as a raw material for producing white pigment, mostly for the paint and plastics industries.
Pig iron is a co-product of titanium dioxide slag production
Ilmenite process
Basic reactions in ilmenite smelting are:
Reduction of FeO from slag
FeO + C → Fe + CO
Partial reduction of TiO2 from slag
TiO2 + 1/2 C → TiO1.5 + 1/2 CO
Watch the video to see the production process at Tizir Titanium & Iron in Senegal and Norway.
More information on pig iron
For further information about pig iron and its advantages in electric arc furnace (EAF) steelmaking and ferrous casting, please see our fact sheets.
For a more detailed study of the blast furnace process, see ‘The Making, Shaping and Treatment of Steel’ (11th Edition) published by the Association of Iron & Steel Technology.
For answers to the most common technical questions on OBMs and their use and effects in different furnaces, see our OBM FAQs.
OBM production
Fact sheets on ore-based metallics
What are Ore-Based Metallics?
An overview of Ore-Based Metallics (OBMs), types, typical benefits in use and general specifications.
Use and Benefits of Ore-Based Metallics
A more detailed coverage of the benefits of using OBMs in EAF steelmaking.
Use of Hot Briquetted Iron (HBI) in the Electric Arc Furnace (EAF) for Steelmaking
Background to and advantages of use of HBI in the EAF and general characteristics.
Use of Hot Briquetted Iron (HBI) in the Basic Oxygen Furnace (BOF) for Steelmaking
Background to and advantages of use of HBI in the BOF and general characteristics.
Use of Hot Briquetted Iron (HBI) in the Blast Furnace (BF) for Hot Metal Production
Background to and reasons for use of HBI in the blast furnace and general characteristics.
Use of Direct Reduced Iron (DRI) in the Electric Arc Furnace (EAF) for Steelmaking
Background to and advantages of use of DRI in the EAF and general characteristics.
Use of Basic Pig Iron in the Electric Arc Furnace (EAF) for Steelmaking
Background to and advantages of use of pig iron in the EAF and general characteristics.
Use of Granulated Pig Iron (GPI) in the Electric Arc Furnace (EAF) for Steelmaking
Background to and advantages of use of GPI in the EAF and general characteristics.
Use of High Purity Pig Iron for Foundries Producing Ductile Iron Castings
Background to and benefits of using HPPI in production of ductile iron castings and general characteristics.
Use of Foundry Pig Iron in Grey Iron Castings
Background to and benefits of using Foundry Pig Iron in production of grey iron castings and general characteristics.
Relevance of Iron Ore to OBMs
Basic information about iron ore and its relevance to production of ore-based metallics.
Value-in-Use of Ore Based Metallics (OBMs) in EAF Steelmaking
An overview of the value-in-use concept and IIMA's model
Types of OBMs
Direct Reduced Iron (DRI)
DRI is the product of the direct reduction of iron ore in the solid state by carbon monoxide and hydrogen derived from natural gas or coal.
More about DRIHot Briquetted Iron (HBI)
HBI is a premium form of DRI that has been compacted at high temperature making it less reactive.
More about HBIPig Iron
Pig iron is the product of smelting iron ore (also ilmenite) with a high-carbon fuel and reductant such as coke, usually with limestone as a flux.
More about Pig IronGranulated Pig Iron (GPI)
Granulating excess pig iron produces a product GPI which can be used as BOF coolant or as feedstock for electric arc furnaces, cupolas and induction furnaces.
More about GPI